This invention relates generally to gas turbine engines and, more particularly, to a method and apparatus for modifying the acceleration schedule in response to certain engine operating parameters.
In the control of jet engine operation it is common to limit the amount of rotor acceleration fuel to eliminate the possibility of compressor stall. This is generally done by way of maintaining an acceleration fuel schedule with controls that sense the factors which determine the compressor's capabilities to maintain stall-free compression ratio and limit the fuel flow accordingly. This acceleration fuel schedule, in addition to preventing excessive acceleration resulting in engine stall or excessive temperature, must permit the engine to meet predetermined acceleration times, and is therefore elevated to as high a degree as possible without exceeding safe limits.
In maintaining an acceleration fuel schedule, a parameter which has found wide acceptance in the art is that of the ratio of fuel flow (WFM) to compressor discharge pressure (CDP). This parameter works very well on most turbojet applications and on those turbofan applications where reasonable amounts of compressor bleed and power extraction are required, and where the flight envelope is somewhat restricted in altitude. However, when an engine is required to operate with large amounts of bleed (either compressor discharge bleed or compressor interstage bleed), then the WFM/CDP acceleration schedule designed to meet the standard criteria does not permit the desired amount of shaft power extraction or yield the required acceleration time during certain flight conditions. In fact, with some combinations of compressor bleed and shaft power extraction, the engine tends to "hang up" and be incapable of acceleration, or the engine's speed may "roll back" as both compressor bleed and the shaft power extraction are increased.
Various solutions have been proposed to accommodate higher power extraction and reduced acceleration time capabilities. The cycle can be designed so as to lower the compressor operating line or the compressor can be designed for a higher stall line and thus a higher acceleration schedule, but either of these results in an overall performance penalty. By another approach, the acceleration schedule can be designed for a high compressor bleed level with associated stall margin loss when the level of bleed is low, but this results in an unacceptable transient stall margin loss during periods of low bleed. By yet another method the acceleration schedule can be designed for a high compressor bleed level with a constant bleed which is either used or dumped overboard, but the dumping overboard of unwanted air results in a severe performance penalty.
It is well known that compressor bleed increases compressor stall margin. However, since the engine must be designed to operate safely during periods when the stall margin is at a minimum, the acceleration schedule is generally chosen to be compatible with a no-bleed condition. Since the bleeding of compressor air causes an increase in the stall margin, an engine operating under such a bleed condition will have a stall margin characteristic which is in excess of that required for safe operation, even during periods of acceleration.
It is, therefore, an object of this invention to provide a fuel control system for an engine adapted to operate at high altitudes with simultaneous high shaft power extraction and compressor bleed.
Another object of this invention is the provision for an acceleration schedule which meets the requirements of predetermined shaft power extraction and acceleration time at variable flight conditions.
Another object of this invention is to obtain increased acceleration capabilities during periods in which air is being bled from the compressor.
These objects and other features and advantages will become more readily apparent upon reference to the following description when taken in conjunction with the appended drawings.